1. Field of the Invention
This invention relates generally to an outboard motor with a plastic lubricant pan, and more particularly to an outboard motor having a plastic lubricant pan and an exhaust conduit surrounded by the lubricant pan.
2. Description of Related Art
An outboard motor typically has a housing unit that can be mounted on an associated watercraft and an internal combustion engine disposed above the housing unit. The housing unit carries a propulsion device such as, for example, a propeller to propel the watercraft. The engine powers the propulsion device with a driveshaft and a propulsion shaft extending through the housing unit.
The engine can be a four-cycle engine. If the outboard motor employs the four-cycle engine, an oil pan or lubricant reservoir is typically disposed below the engine within the housing unit. The oil pan accumulates lubricant oil that has circulated in the engine for lubrication of various moving engine parts. A portion of such oil pans for outboard motors are made of a plastic material, because a plastic oil pan is durable against salt content of seawater, acids produced from exhaust gases and water, and the like. For example, Japanese Patent Publication No. 9-79486 discloses such a plastic oil pan.
The outboard motor also has an exhaust system to route the exhaust gases from the engine through the housing unit to an external location . Typically, internal exhaust sections are provided within the housing unit and the majority of exhaust gases are discharged through the internal exhaust sections to a body of water that surrounds the outboard motor. The internal exhaust sections include an exhaust conduit disposed below the engine.
Because the exhaust conduit extends through a space where the oil pan is disposed, typically the oil pan is generally configured as a doughnut shape such that the exhaust conduit passes through the oil pan. In other words, the oil pan generally surrounds the exhaust conduit. In some arrangements, a center portion of the oil pan can form an exhaust passage that communicates with the exhaust conduit.
The exhaust system produces heat due to an air/fuel charge that burns in one or more combustion chamber of the engine. The burnt charges, i.e., hot exhaust gases, pass through the exhaust system. In addition, it is possible that flames from the combustion chamber(s) can reach the exhaust conduit.
If the outboard motor employs the foregoing plastic oil pan, a relatively large space is necessary between the oil pan and the exhaust conduit to isolate the oil pan from the exhaust conduit. In addition, if a portion of the plastic oil pan forms a part of exhaust passages as discussed above, the oil pan portion needs to be thicker than that of an oil pan made of metal. A plastic oil pan needs to be thicker because a plastic oil pan that is too thin can be relatively easily deformed or melted by the heat of exhaust gases or flames. The oil pan cannot extend outward beyond a pre-determined area because the housing unit that embraces the oil pan should be kept small to make the outboard motor compact. An oil pan that extends vertically beyond a pre-determined area also is not suitable because additional parts of the internal exhaust sections such as an exhaust expansion pipe cannot have sufficient volume to reduce the exhaust energy. As a result, a substantial capacity of the oil pan for the lubricant oil is normally smaller than that of an oil pan using a metal material.